1. Field of the Invention
The present invention relates to a photothermographic material preferably used in the field of films for medical diagnosis, the field of films for graphic arts, or the like.
2. Description of the Related Art
In recent years, in the medical field and the graphic arts field, there has been a strong desire for providing a dry photographic process from the viewpoints of protecting the environment and economy of space. Further, the development of digitization in these fields has resulted in the rapid development of systems in which image information is captured and stored in a computer, and then when necessary processed and output by transmitting it to a desired location. Here the image information is output onto a photosensitive material using a laser image setter or a laser imager, and developed to form an image at the location. It is necessary for the photosensitive material to be able to record an image with high-intensity laser exposure and that a clear black-tone image with a high resolution and sharpness can be formed. While various kinds of hard copy systems using pigments or dyes, such as ink-jet printers or electrophotographic systems, have been distributed as general image forming systems using such digital imaging recording materials, images on the digital imaging recording materials obtained by such general image forming systems are insufficient in terms of the image quality (sharpness, granularity, gradation, and tone) needed for medical images used in making diagnoses, and high recording speeds (sensitivity). These kinds of digital imaging recording materials have not reached a level at which they can replace medical silver halide film processed with conventional wet development.
Photothermographic materials utilizing organic silver salts are already known. Photothermographic materials have an image forming layer in which a reducible silver salt (for example, an organic silver salt), a photosensitive silver halide, and if necessary, a toner for controlling the color tone of developed silver images are dispersed in a binder.
Photothermographic materials form black silver images by being heated to a high temperature (for example, 80° C. or higher) after imagewise exposure to cause an oxidation-reduction reaction between a silver halide or a reducible silver salt (functioning as an oxidizing agent) and a reducing agent. The oxidation-reduction reaction is accelerated by the catalytic action of a latent image on the silver halide generated by exposure. As a result, a black silver image is formed in the exposed region. Photothermographic materials have been described in many documents, and the Fuji Medical Dry Imager FM-DPL is an example of a practical medical image forming system using a photothermographic material that has been marketed.
These photothermographic materials utilizing an organic silver salt have a great characteristic of containing all components necessary for image formation in the film in advance and being capable of forming images only by heating. However, on the other hand, there are many problems to be solved.
Photothermographic materials do not require the processing solutions used in conventional wet processing in the case of silver halide photosensitive materials, and have an advantage in that processing can be carried out easily and rapidly. However, there are still problems to be solved with respect to photothermographic materials, which do not occur in conventional wet processing in the case of silver halide photosensitive materials. One of them is the problem of decolorization of dyes. Silver halide photosensitive materials commonly incorporate dyes in order to provide a light filter and prevent halation or irradiation therein. The added dyes function during imagewise exposure. In the case where the dyes have a spectral light absorption in the visible region, if the dyes remain in a photosensitive material after performing their function, the formed images may be colored by the dyes, and image quality may be damaged. Therefore the residual dyes are preferably removed from the photosensitive materials during the developing process. In a wet developing process, the residual dyes can be removed easily from the photosensitive materials by a processing solution. On the other hand, in the case of the photothermographic material, it is a significant task to remove the residual dyes.
More specifically, in order to attain images with a good degree of sharpness, the incorporation of dyes is very important for photothermographic materials exposed by a laser beam to provide sufficient antihalation and anti-irradiation effects at the wavelength used for the imagewise exposure. As for the wavelength of a laser beam used for the exposure, a wide range of wavelength regions such as the near infrared region, the infrared region, or the visible region from red to blue can be applied.
For photothermographic materials exposed with either a near infrared or an infrared laser beam, Japanese Patent Application Laid-Open (JP-A) Nos. 9-146220 and 11-228698 disclose photothermographic materials which practically require no color bleaching mechanism therein due to use of a dye which has an absorption maximum within the near infrared regions outside of visual sensitivity, a narrow half band width, and little light absorption within the visual region. All patents, patent publications, and non-patent literature cited in this specification are hereby expressly incorporated by reference herein.
For photothermographic materials which are subjected to imagewise exposure with a laser beam having a wavelength within the visible region of blue to red, it is preferable to incorporate some kind of decoloring reaction mechanism.
A method for decoloring dyes by way of heating during a thermal developing process has been proposed. For example, U.S. Pat. No. 5,135,842 discloses a method for decoloring polymethine dyes of a specific structure by heating. Moreover, U.S. Pat. Nos. 5,314,795, 5,324,627, and 5,384,237 disclose methods in which polymethine dyes are decolorized by heating using a carbanion generating agent.
However, the decoloring mechanisms described above often bring about problems such as incomplete decoloring of dyes or dye decolorization during storage of photothermographic materials due to the insufficient stability of dye occurring after bleaching ability has been enhanced. Especially, in photothermographic materials used in medical diagnosis, high sharpness and preferable image tone are required. Furthermore, demand has increased for image forming methods used for processing photothermographic materials rapidly at a higher speed within a short time. Particularly in uses for medical treatment, there is a strong desire for rapid diagnosis.